This invention relates to mobile or portable cellular communication systems, and more particularly to a compact foldable antenna apparatus for use with mobile or portable subscriber units.
Code division multiple access (CDMA) communication systems provide wireless communications between a base station and one or more mobile or portable subscriber units. The base station is typically a computer-controlled set of transceivers that are interconnected to a land-based public switched telephone network (PSTN). The base station further includes an antenna apparatus for sending forward link radio frequency signals to the mobile subscriber units and for receiving reverse link radio frequency signals transmitted from each mobile unit. Each mobile subscriber unit also contains an antenna apparatus for the reception of the forward link signals and for the transmission of the reverse link signals. A typical mobile subscriber unit is a digital cellular telephone handset or a personal computer coupled to a cellular modem. In such systems, multiple mobile subscriber units may transmit and receive signals on the same center frequency, but unique modulation codes distinguish the signals sent to or received from individual subscriber units.
In addition to CDMA, other wireless access techniques employed for communications between a base station and one or more portable or mobile units include those described by the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard and the industry-developed Bluetooth standard. All such wireless communications techniques require the use of an antenna at both the receiving and transmitting end. It is well-known by experts in the field that increasing the antenna gain in any wireless communication system has beneficial affects on wireless systems performance.
A common antenna for transmitting and receiving signals at a mobile subscriber unit is a monopole antenna (or any other antenna with an omnidirectional radiation pattern) A monopole consists of a single wire or antenna element that is coupled to a transceiver within the subscriber unit. Analog or digital information for transmission from the subscriber unit is input to the transceiver where it is modulated onto a carrier signal at a frequency using a modulation code (i.e., in a CDMA system) assigned to that subscriber unit. The modulated carrier signal is transmitted from the subscriber unit to the base station. Forward link signals received by the subscriber unit are demodulated by the transceiver and supplied to processing circuitry within the subscriber unit.
The signal transmitted from a monopole antenna is omnidirectional in nature. That is, the signal is sent with approximately the same signal strength in all directions in a generally horizontal plane. Reception of a signal with a monopole antenna element is likewise omnidirectional. A monopole antenna does not differentiate in its ability to detect a signal in one azimuth direction versus detection of the same or a different signal coming from another azimuth direction. Also, a monopole antenna does not produce significant radiation in the elevation direction. The antenna pattern is commonly referred to as a donut shape with the antenna element located at the center of the donut hole.
A second type of antenna that may be used by mobile subscriber units is described in U.S. Pat. No. 5,617,102. The directional antenna comprises two antenna elements mounted on the outer case of a laptop computer, for example. A phase shifter attached to each element imparts a phase angle delay to the input signal, thereby modifying the antenna pattern (which applies to both the receive and transmit modes) to provide a concentrated signal or beam in the selected direction. Concentrating the beam increases the antenna gain and directivity. The dual element antenna of the cited patent thereby directs the transmitted signal into predetermined sectors or directions to accommodate for changes in orientation of the subscriber unit relative to the base station, thereby minimizing signal loss due to the orientation change. In accordance with the antenna reciprocity theorem, the antenna receive characteristics are similarly effected by the use of the phase shifters.
CDMA cellular systems are interference limited systems. That is, as more mobile or portable subscriber units become active in a cell and in adjacent cells, frequency interference increases and thus bit error rates also increase. To maintain signal and system integrity in the face of increasing error rates, the system operator decreases the maximum data rate allowable for one or more users, or decreases the number of active subscriber units, which thereby clears the airwaves of potential interference. For instance, to increase the maximum available data rate by a factor of two, the number of active mobile subscriber units is halved. However, this technique cannot generally be employed to increase data rates due to the lack of service priority assignments to the subscribers. Finally, it is also possible to avert excessive interference by using directive antennas at both (or either) the base station and the portable units.
Typically, a directive antenna beam pattern is achieved through the use of a phased array antenna. The phased array is electronically scanned or steered to the desired direction by controlling the phase angle of the input signal to each antenna element. However, phased array antennas suffer decreased efficiency and gain as the element spacing becomes electrically small when compared to the wavelength of the received or transmitted signal. When such an antenna is used in conjunction with a portable or mobile subscriber unit, generally the antenna array spacing is relatively small and thus antenna performance is correspondingly compromised.
In a communication system in which portable or mobile units communicate with a base station, such as a CDMA communication system, the portable or mobile unit is typically a hand-held device or a relatively small device, such as, for instance, the size of a laptop computer. In some embodiments, the antenna is inside or protrudes from the devices housing or enclosure. For example, cellular telephone hand sets utilize either an internal patch antenna or a protruding monopole or dipole antenna. A larger portable device, such as a laptop computer, may have the antenna or antenna array mounted in a separate enclosure or integrated into the laptop housing. A separately-enclosed antenna may be cumbersome for the user to manage as the communications device is carried from one location to another. While integrated antennas overcome this disadvantage, such antennas, except for a patch antenna, generally are in the form of protrusions from the communications device. These protrusions can be broken or damaged, as the device is moved from one location to another. Even minor damage to a protruding antenna can drastically alter its operating characteristics.
Problems of the prior art
Several considerations must be taken into account in integrating a wireless-network antenna into an enclosure, whether the enclosure comprises a unit separate from the communications device or the housing of the communications device itself. In designing the antenna and its associated enclosure, careful consideration must be given to the antenna electrical characteristics so that signals propagating over the wireless link satisfy pre-determined system standards, such as, the bit error rate, signal-to-noise ratio or signal-to-noise-plus-interference ratio. The electrical properties of the antenna, as influenced by the antenna physical parameters, are discussed further herein below.
The antenna must also exhibit certain mechanical characteristics to satisfy user needs and meet the required electrical performance. The antenna length, or the length of each element of an antenna array, depends on the received and transmitted signal frequencies. If the antenna is configured as a monopole, the length is typically a quarter wavelength of the signal frequency. For operation at 800 MHz (one of the wireless frequency bands), a quarter-wavelength monopole is 3.7 inches long. The length of a half-wavelength dipole is 7.4 inches.
The antenna must further present an aesthetically pleasing appearance to the user. If the antenna is deployable from the communications device, sufficient volume within the communications device must be allocated to the stored antenna and its peripheral components. But since the communications device is used in mobile or portable service, the device must remain relative small and light with a shape that allows it to be easily carried. The antenna deployment mechanism must be mechanically simple and reliable. For those antennas housed in an enclosure separate form the communications device, the connection mechanism between the antenna and the communications device must be reliable and simple.
Not only are the electrical, mechanical and aesthetic properties of the antenna important, but it must also overcome unique performance problems in the wireless environment. One such problem is called multipath fading. In multipath fading, a radio frequency signal transmitted from a sender (either a base station or mobile subscriber unit) may encounter interference in route to the intended receiver. The signal may, for example, be reflected from objects, such as buildings, thereby directing a reflected version of the original signal to the receiver. In such instances, two versions of the same radio frequency signal are received; the original version and a reflected version. Each received signal is at the same frequency, but the reflected signal may be out of phase with the original due to the reflection and consequent differential transmission path length to the receiver. As a result, the original and reflected signals may partially cancel each other out (destructive interference), resulting in fading or dropouts in the received signal.
Single element antennas are highly susceptible to multipath fading. A single element antenna cannot determine the direction from which a transmitted signal is sent and therefore cannot be tuned to more accurately detect and receive a transmitted signal. Its directional pattern is fixed by the physical structure of the antenna components. Only the antenna position and orientation can be changed in an effort to obviate the multipath fading effects.
The dual element antenna described in the aforementioned patent reference is also susceptible to multipath fading due to the symmetrical and opposing nature of the hemispherical lobes of the antenna pattern. Since the antenna pattern lobes are more or less symmetrical and opposite from one another, a signal reflected to the back side of the antenna may have the same received power as a signal received at the front. That is, if the transmitted signal reflects from an object beyond or behind the intended receiver and then reflects into the back side of the antenna, it will interfere with the signal received directly from the source, where the phase difference in the two signals creates destructive interference due to multipath fading.
Another problem present in cellular communication systems is inter-cell signal interference. Most cellular systems are divided into individual cells, with each cell having a base station located at its center. The placement of each base station is arranged such that neighboring base stations are located at approximately sixty degree intervals from each other. Each cell may be viewed as a six sided polygon with a base station at the center. The edges of each cell abut the neighboring cells and a group of cells form a honeycomb-like pattern. The distance from the edge of a cell to its base station is typically driven by the minimum power required to transmit an acceptable signal from a mobile subscriber unit located near the edge of the cell to that cell""s base station (i.e., the power required to transmit an acceptable signal a distance equal to the radius of one cell).
Intercell interference occurs when a mobile subscriber unit near the edge of one cell transmits a signal that crosses over the edge into a neighboring cell and interferes with communications taking place within the neighboring cell. Typically, signals in neighboring cells on the same or closely spaced frequencies cause intercell interference. The problem of intercell interference is compounded by the fact that subscriber units near the edges of a cell typically transmit at higher power levels so that the transmitted signals can be effectively received by the intended base station located at the cell center. Also, the signal from another mobile subscriber unit located beyond or behind the intended receiver may arrive at the base station at the same power level, representing additional interference.
The intercell interference problem is exacerbated in CDMA systems since the subscriber units in adjacent cells typically transmit on the same carrier or center frequency. For example, two subscriber units in adjacent cells operating at the same carrier frequency but transmitting to different base stations interfere with each other if both signals are received at one of the base stations. One signal appears as noise relative to the other. The degree of interference and the receiver""s ability to detect and demodulate the intended signal is also influenced by the power level at which the subscriber units are operating. If one of the subscriber units is situated at the edge of a cell, it transmits at a higher power level, relative to other units within its cell and the adjacent cell, to reach the intended base station. But, its signal is also received by the unintended base station, i.e., the base station in the adjacent cell. Depending on the relative power level of two same-carrier frequency signals received at the unintended base station, it may not be able to properly differentiate a signal transmitted from within its cell from the signal transmitted from the adjacent cell. A mechanism is required to reduce the subscriber unit antenna""s apparent field of view, which can have a marked effect on the operation of the reverse link (subscriber to base) by reducing the number of interfering transmissions received at a base station. A similar improvement in the antenna pattern for the forward link, allows a reduction in the transmitted signal power to achieve a desired receive signal quality.
In summary, it is clear that in the wireless communications technology, it is of utmost importance to maximize antenna performance, while minimizing size and manufacturing complexity.
Brief Description of the Present Invention
A directional antenna having a center element and a plurality of radial elements spaced apart from the center element, further including a deformable or pivotable joint between the center element and each radial element providing for pivotable displacement of the radial elements relative to the center element, such that the entire antenna including the center element and the radial elements are foldable into a flattened or stacked orientation. If the radial elements are formed on deformable or flexural material, they are affixed to the center element, creating a deformable union there between, with an adhesive or mating soldered vias. Alternatively, if the radial elements are formed on a rigid dielectric material, a piece of deformable or flexural material is interposed between the rigid dielectric material of the radial elements and the center element to form the deformable union therebetween. The piece of deformable or flexural material can be affixed to both the rigid dielectric material of the radial elements and the center element by several techniques that are known in the art, including use of an adhesive material or by mated solderable vias.
Essentially, each radial element comprises a first elongated dielectric substrate section on which a conductive element is disposed, and a second elongated section perpendicular to the first elongated section, and forming an interconnecting arm between the first elongated section and the center element. Likewise, the center element comprises an elongated dielectric substrate on which a conductive element is disposed. The distal end of the interconnecting arm is joined to the elongated dielectric substrate carrying the center element by way of the deformable union as discussed above.
When deployed, the antenna array provides advantageous directional characteristics. In particular, in one embodiment each of the radial elements is controllably operated in either a reflector or director mode, with respect to signals transmitted from or received by the center element. Further, the center element and the plurality of radial elements are each disposed on a dielectric substrate, wherein one or more of the dielectric substrates is configured to carry microelectronic components for controlling operational aspects of the center element and the radial elements. Included among these microelectronic components are certain switches connected between an upper conductive portion and a lower conductive portion of the conductive element for each one of the plurality of radial elements. Signals supplied to the antenna array control the active switches for shorting or opening the connection between the upper portion and the lower linear portion of each radial element, to achieve either the directive or the reflective operational state. In yet another embodiment the center element is absent and the radial elements are controllably operated as a phased array antenna.